Wind Friction Speeds for Particle Movement - Semantic Scholar
Vol.
3, No. 8
Geophysical
MARS:
WIND
Research
FRICTION
SPEEDS
Letters
August
FOR PARTICLE
1976
MOVEMENT
R. Greeley
University
of Santa Clara Moffett
and NASA-AmesResearch Center
Field, B.
Department of Mechanical
White
Engineering, Davis,
of Aerospace
J.
Space Sciences
Iversen
Iowa
Field,
speeds
require free-stream winds of 50 to 135 m/sec,
depending on the character of the surface and the atmospheric conditions. The corresponding wind speeds at the height of the Viking lander meteorology instrument would be about a factor the
free
stream wind
size most easily
conditions.
grains;
thus,
most research
speed.
sented
moved by winds
as
"threshold"
hold friction
and smaller than this (at least down to about 5 •m) require stronger winds to initiate movement. The results presented here are in general
of a particle
agreementwith previously reported values of V, t
where
ß is
fluid
density.
particle.
eolian activity
fr øm Mariner
of
dust
9 have
storms
on
that
relate
to the diameter
thres-
of the
The thresholdfriction velocity (V,t) is the friction
speed (V,) needed
grain movement. V, = • the
surface
shear
stress
V, is directly
and
P is
proportional
the
to
the wind speed for a neutral adiabatic atmosphere; at an ambient pressure of 5 mb, the free stream velocity is about 17 times the friction speed in the wind tunnel.
IntrOduction
observations
curves
velocity
to initiate
for particles 12 •m to 300 •m derived from one atmosphere tests, but are inconsistent with values for particles larger than about 300 •m.
Earth-based
has concentrated
the conditions (wind speeds, etc.) needed to initiate saltation. Results typically are pre-
on Mars is about 160 •m; particles both larger
Mars and results
Center
CA 94035
Winds transport particles by saltation (bouncing grains), surface creep, and suspension.' Particles on Earth are most easily moved by saltation, with both surface creep and suspension resulting primarily from the impact of saltating
the minimum V,t is about2.5 m/sec,whichwould
than
NASA-Ames Research
near-martian
a low pressure boundary layer wind tunnel at an atmospheric pressure of 5.3 mb. The results imply that for comparable pressures on Mars,
of two less
University
This report presents preliminary results of wind tunnel simulations of particle movement Under
(V,t) for particle movement weredeterminedin
The particle
Iowa State
50011
Pollack
Division,
Moffett
threshold
CA 94035
Engineering,
Ames,
Wind friction
at Davis
Leach
Field, J.
Abstract.
of California
of Santa Clara and NASA-AmesResearch Center Moffett
Department
University
CA 95616
R.
University
CA 94035
on
On Earth there is an optimum grain size for movement by minimum Winds, with stronger winds needed for movement of both larger and' smaller grains. The reason for the "upturn" in the threshold curve (fig. 1) for the smaller grains is poorly known, but appears to be related primarily to interparticle forces (e.g., cohesion),
shown that
is .an important surface-modify'
ing process on Mars and have prompted interest in the nature of wind-blown particles. Interest in eolian processes is heightened by the Viking mission that will place two spacecraft in orbit and two landers on Mars this summer. Knowledge of the physics of particle movement in the martian environment is required to understand the generation of dust storms, rates of erosion and deposition, and other geological processes.
as well as aerodynamic effects
(Iversen
et al.,
1976).
Nearly
all
predictions
of martian
threshold
friction speeds (Sagan and Pollack, 1969, Arvidson, 1972, Greeley et al., 1974, and others) are
Copyright 1976 by the American Geophysical Union
based on wind tunnel 417
experiments
conducted at
418
Greeley 6
Wind Friction
Speeds
-
>-
nominal Mars case), the wind velocity
•X/ IVERSEN etal., 1976
O
O
et al.:
creased
until
o
particles.
I
I
I
I
I IIII
I
10
I
1.
Threshold
le.•z et al.,
1000
DIAMETER,/•m
friction
velocities
ob-
Also
shown
is
the
speed for the particle
stream
tested (Gree-
threshold
Materials
used in
the
threshold
tests
are shownin Table 1.
and their
Although
there is no single material that satisfies test parameters for simulating Mars, this of samples is considered a suitable "first mate," based on the following rationale:
(white circles; size range indicated by bars). Black symbols indicate samples that were run in a dried state; squares indicate samples of sizes.
free
1974).
characteristics
tained in the wind tunnel at an atmospheric pressure of 5.3 mb for walnut shell particles
mixed
of the measured
I I I III
100
PARTICLE
Figure
I
The value
hold friction
ELEY et al " 19
2
slowly in-
was observed.
wind velocity at threshold was then correlated with the boundary layer profile to obtain a thres-
¸ SAGAN AND BAGNOLD, 19757• -r I--
threshold
The criteria for determining threshold follow those of Bagnold (1941); threshold is defined as the movement of particles over the entire bed, rather than the movement of only a few individual
_o'• 4
.
grain
curve
on Mars is
0.38
less
required
force
ment.
of Iversen et al. (1976), based on 1 bar wind tunnel experiments extrapolating the low pressure conditions and taking into account interparticle forces and lift coefficients; the
Mars, Mars
For
that
tests
grains should
Earth,
which
to initiate
conducted
0.38 be
of
on
Gravity
results
particle Earth
all suite esti-
to
in
movesimulate
as dense as those expected on
used
to
offset
the
difference
in
1 bar and extrapolated to the martian environment. The most important parameter in the extrapolation is atmospheric pressure -- the low pres-
gravity. Given the known geological environment on Mars, wind blown particles with specific gravities of 2.6 to 3.0 are reasonable; for threshold simulations on Earth, the corresponding material specific gravities should be •1.0 to 1.2. Walnut shells were selected as appropriate particles for the primary series of tests because they have a specific gravity of 1.1, can be ground and sorted into different sizes, and the particles have about the same angular shape as natural eolian material
sures on Mmrs (•1 to 10 mb) require substantially stronger winds to move particles than on Earth. The basic physics of wind-blown particles is pre-
(sand and loess). Because the shape of the ground walnut shells is comparable to natural aeolian material, the lift and drag coefficients should
sented in the classic work by Bagnold (1941), but
also be comparable.
curve of Greeley et al. these
parameters
into
(1974)
does not take
account.
the relative
importance
of individual
such as lift
coefficients and cohesion is poorly
known. Yet, an understanding is critical for extrapolations
parameters
of these parameters to Mars. Although
An important factor in particle
threshold is
the effect of cohesion from adsorbed moisture and from other surface forces. This effect increases
someexperiments have been conducted at 1 bar
with sinroller particles
which take these parameters into account (Iversen et al.., 1976), current extrapolations to Mars
mass ratio increases. To determine if cohesion from moisture is the primary cause for the "upturn" in the threshold curve for small particles
produce widely divergent results because of uncertainties in knowledge of forces on small particles at low pressure. Wind
Tunnel
Simulations
lished
at
NASA-Ames
Research
Center
in
which
threshold experiments can be conducted under conditions approximating those of Mars except for the differences in gravity and atmospheric composition. Experiments were run by placing a patch of grains in the tunnel test section and, under the desired atmospheric pressure (5.3 mb for the TABLE
MATERIALS MATERIAL
and to assess the effect at low pressure, several particle tests were conducted in which the samples were dried by heating and then tested in the tunnel.
Because of the uncertainties in particle movement on Mars, a low pressure wind tunnel was estab-
SPECIFIC
since the surface area-to-
The
test
bed
of
the
tunnel
was heated
Because not all parameters involved in martian eolian processes can be simulated in wind tunnel tests on Earth, it is necessary to use a combination of theory and wind tunnel results for extrapolation to Mars. Before the extrapolation is made, however, as much of the theory as possible should be tested. The expressions derived by 1
USED FOR THRESHOLD TESTS
GRAVITY
SIZE
RANGE, •4m
SHAPE
,
Walnut shell Calcium carbonate
1.1 2.7
Glass microspheres
2.7
Talc
3.0
to
the temperature that kept the sample dry but did not affect the boundary layer.
20-700 20-500 40-300 5-
20
angular subrhombohedra
1
spherical
angular
to platy
Greeley
et
al.:
Wind Friction
Speeds
419
Iversen et al., (1976) for extrapolation to Mars take into account interparticle forces, surface roughness, lift coefficients, and other parameters, but are based on wind tunnel tests performed
at
1 bar
and
contain
uncertainties
as
-
110
100
30(
to
25(
the behavior of particles under low pressure. These expressions were tested against the wind tunnel experiments performed at 5.3 mb (fig. 1) and found to be in good agreement, particularly in the critical particle size range of 30 to 200 •m. The cause of the divergence of the test results and the theory for large particles, however, is unresolved. Future experiments will focus on both larger and smaller particles than those used in these experiments.
7o
20(
•
2.5mb 60 "•
/• 15( 5o •
•:
50mb
•
10.0mb
4o :>•
1 o(
50
Results
and
Conclusions --
Figure 1 shows threshold results performed at low pressure, but with Earth air; figure 2 shows
threshold
wind
friction
velocities
o
to
Mars
based
on the
results
of
for
Mars:
over a flat
1.
Minimum threshold friction
layer)
velocity
conditions smooth this cor-
stream (above the boundary
of about 125 m/sec.
Corres-
ponding wind speeds at the height of the Viking meteorology instrument would be about half the free stream wind speed. These values could be substantially lower, however, depending on the nature of the surface. For example, if large non-erodible
present,
elements
such
then the same V,•
as
cobbles
I
III
o
0
1ooo
(Vgt) basedon atmospheric modelsby Pollack (1976); Case 1 is for winds blowing
cobbles
minimum V,t is about2.5 m/sec. Usingthe
responds to a free
I
smooth surface composed of erod-
ible grains, Case 2 is for a surface containing
For a
of 5 rob, the
expressions for martian atmospheric (Pollack et al., 1976a) for a flat, surface composed of loose particles,
I
(above the boundary layer) wind velocities
for
pressure
I
scales on right are equivalent free stream et al.
surface
I
Figure 3. Martian particle threshold curves as a function of particle size at three pressures. Scale on left is V,• in cm/sec, two
Iversen
speed.
I I • •1 1oo
et al. (1976) using the appropriate values for kinematic viscosity and density for the martian atmosphere. Several results are significant
nominal martian
I
PARTICLE DIAMETER,
four atmospheric pressures, demonstrating the dependence of threshold on pressure. Figure 3 is an extrapolation of the wind tunnel results
I
lO
10
are
could be generated
by windsone third to oneChalfthat given
and small
boulders.
above (fig. 3). These minimum free stream winds are significantly higher than typical speeds on Mars obtained from general circulation calculations; however, such winds are predicted to occur occasionally (Pollack et al., 1976b). 2. Small particle threshold. Sagan and Baghold (1975) recently extrapolated particle threshold velocities to Mars based on experiments of cohesion-free particle transport. They suggested that particle cohesion due to impact vitrification, vacuum sintering, and adsorbed thin films of water
might
particles
be absent
on Mars,
and that
small
(• 1 •m) might be more easily moved than
100 •m particles. Low pressure wind tunnel results performed with dried samples and in the ab-
7 mb
sence of impact
5.3
ing still show an "upturn" in the threshold curve for small particles (fig. 1). Thus, if grain
6.5
movement is observed on the martian surface,
vitrification
and vacuum sinter-
minimum V,t at 5 mbis 2.5 m/secand wind speeds near the s6rface on the order of 25 to 75 m/sec should be expected. These estimates are based on particle threshold taking place on a flat surface; local topography such as raised-rim
7.9
10.5
craters
could initiate
threshold
at
lower wind
speeds as a result of local vortiCies, cussed by Greeley et. al. (1974).
3. 2 10
I
I
I
I
I I I I I
I
I
•
• • • • •1
100
1000
PARTICLE DIAMETER,/•m
on
pressure.
velocities
The particle
size
most easily moved (at a•m minimum Mars appears to be about 160 (fig. V•f) oron nearly twice as large as the optimum particle
on Earth.
size
Whensaltation begins, both larger
and smaller particles
Figure 2. Threshold curves for particles obtained in the wind tunnel for four pressures, showing the dependence of threshold
Optimumpa.rticle size.
as dis-
will
be set into motion.
The values of threshold speeds presented here represent the first series of experiments performedin a large, low pressure, boundary
420
Greeley
et al.:
layer wind tunnel and should be regarded as preliminary. Further refinements of the threshold
curve
and larger
and
particles
Acknowledgement.
extension
to
both
smaller
are in progress.
This work was supported
Re ferences
Arvidson, R. E., 1972. Aeolian processes on Mars: Erosive velocities, settling velocities, and yellow clouds. Geol. Soc. Am.
Bull. 83, 1503-1508. Bagnold, R. A., 1941. The Physics of Blown Sand
and
R.,
Desert
Dunes.
J. D. Iversen,
London:
Methuen.
J. B. Pollack,
N. Udovich, and B. White, 1974. Wind tunnel studies of martian aeolian processes. Proc. R. Soc. London A. 341, 331-360.
Iversen,
J. D.,
J. B. Pollack,
B.
Speeds
R. White,
1976.
Saltation
threshold
R. Greeley,
on
Mars: The effect of interparticle force, surface roughness, and low atmospheric
density. In press, Icarus. Pollack, J. B., R. Haberle, R. Greeley,
by the Planetary Geology Program Office, National Aeronautics and Space Administration.
Greeley,
Wind Friction
Iversen,
1976a.
Estimates
of
J.
the wind
speeds required for particle motion on Mars. In press, Icarus. Pollack, J. B., C. B. Leovy, Y. H. Mintz, and W. VanCamp, 1976b. Winds on Mars during the Viking season..predictions based on a general circulation model with
topography.
Submitted to Geophys. Res.
Letters.
Sagan,
C. and J.
B. Pollack,
1969.
Wind-
blown dust on Mars. Nature, 223, 791-794. Sagan, C. and R. A. Bagnold, 1975. Fluid transport on Earth and aeolian transport on Mars.
Icarus
(Received accepted
26,
209-218.
June June
9, 1976; 23, 1976.)
3, No. 8
Geophysical
MARS:
WIND
Research
FRICTION
SPEEDS
Letters
August
FOR PARTICLE
1976
MOVEMENT
R. Greeley
University
of Santa Clara Moffett
and NASA-AmesResearch Center
Field, B.
Department of Mechanical
White
Engineering, Davis,
of Aerospace
J.
Space Sciences
Iversen
Iowa
Field,
speeds
require free-stream winds of 50 to 135 m/sec,
depending on the character of the surface and the atmospheric conditions. The corresponding wind speeds at the height of the Viking lander meteorology instrument would be about a factor the
free
stream wind
size most easily
conditions.
grains;
thus,
most research
speed.
sented
moved by winds
as
"threshold"
hold friction
and smaller than this (at least down to about 5 •m) require stronger winds to initiate movement. The results presented here are in general
of a particle
agreementwith previously reported values of V, t
where
ß is
fluid
density.
particle.
eolian activity
fr øm Mariner
of
dust
9 have
storms
on
that
relate
to the diameter
thres-
of the
The thresholdfriction velocity (V,t) is the friction
speed (V,) needed
grain movement. V, = • the
surface
shear
stress
V, is directly
and
P is
proportional
the
to
the wind speed for a neutral adiabatic atmosphere; at an ambient pressure of 5 mb, the free stream velocity is about 17 times the friction speed in the wind tunnel.
IntrOduction
observations
curves
velocity
to initiate
for particles 12 •m to 300 •m derived from one atmosphere tests, but are inconsistent with values for particles larger than about 300 •m.
Earth-based
has concentrated
the conditions (wind speeds, etc.) needed to initiate saltation. Results typically are pre-
on Mars is about 160 •m; particles both larger
Mars and results
Center
CA 94035
Winds transport particles by saltation (bouncing grains), surface creep, and suspension.' Particles on Earth are most easily moved by saltation, with both surface creep and suspension resulting primarily from the impact of saltating
the minimum V,t is about2.5 m/sec,whichwould
than
NASA-Ames Research
near-martian
a low pressure boundary layer wind tunnel at an atmospheric pressure of 5.3 mb. The results imply that for comparable pressures on Mars,
of two less
University
This report presents preliminary results of wind tunnel simulations of particle movement Under
(V,t) for particle movement weredeterminedin
The particle
Iowa State
50011
Pollack
Division,
Moffett
threshold
CA 94035
Engineering,
Ames,
Wind friction
at Davis
Leach
Field, J.
Abstract.
of California
of Santa Clara and NASA-AmesResearch Center Moffett
Department
University
CA 95616
R.
University
CA 94035
on
On Earth there is an optimum grain size for movement by minimum Winds, with stronger winds needed for movement of both larger and' smaller grains. The reason for the "upturn" in the threshold curve (fig. 1) for the smaller grains is poorly known, but appears to be related primarily to interparticle forces (e.g., cohesion),
shown that
is .an important surface-modify'
ing process on Mars and have prompted interest in the nature of wind-blown particles. Interest in eolian processes is heightened by the Viking mission that will place two spacecraft in orbit and two landers on Mars this summer. Knowledge of the physics of particle movement in the martian environment is required to understand the generation of dust storms, rates of erosion and deposition, and other geological processes.
as well as aerodynamic effects
(Iversen
et al.,
1976).
Nearly
all
predictions
of martian
threshold
friction speeds (Sagan and Pollack, 1969, Arvidson, 1972, Greeley et al., 1974, and others) are
Copyright 1976 by the American Geophysical Union
based on wind tunnel 417
experiments
conducted at
418
Greeley 6
Wind Friction
Speeds
-
>-
nominal Mars case), the wind velocity
•X/ IVERSEN etal., 1976
O
O
et al.:
creased
until
o
particles.
I
I
I
I
I IIII
I
10
I
1.
Threshold
le.•z et al.,
1000
DIAMETER,/•m
friction
velocities
ob-
Also
shown
is
the
speed for the particle
stream
tested (Gree-
threshold
Materials
used in
the
threshold
tests
are shownin Table 1.
and their
Although
there is no single material that satisfies test parameters for simulating Mars, this of samples is considered a suitable "first mate," based on the following rationale:
(white circles; size range indicated by bars). Black symbols indicate samples that were run in a dried state; squares indicate samples of sizes.
free
1974).
characteristics
tained in the wind tunnel at an atmospheric pressure of 5.3 mb for walnut shell particles
mixed
of the measured
I I I III
100
PARTICLE
Figure
I
The value
hold friction
ELEY et al " 19
2
slowly in-
was observed.
wind velocity at threshold was then correlated with the boundary layer profile to obtain a thres-
¸ SAGAN AND BAGNOLD, 19757• -r I--
threshold
The criteria for determining threshold follow those of Bagnold (1941); threshold is defined as the movement of particles over the entire bed, rather than the movement of only a few individual
_o'• 4
.
grain
curve
on Mars is
0.38
less
required
force
ment.
of Iversen et al. (1976), based on 1 bar wind tunnel experiments extrapolating the low pressure conditions and taking into account interparticle forces and lift coefficients; the
Mars, Mars
For
that
tests
grains should
Earth,
which
to initiate
conducted
0.38 be
of
on
Gravity
results
particle Earth
all suite esti-
to
in
movesimulate
as dense as those expected on
used
to
offset
the
difference
in
1 bar and extrapolated to the martian environment. The most important parameter in the extrapolation is atmospheric pressure -- the low pres-
gravity. Given the known geological environment on Mars, wind blown particles with specific gravities of 2.6 to 3.0 are reasonable; for threshold simulations on Earth, the corresponding material specific gravities should be •1.0 to 1.2. Walnut shells were selected as appropriate particles for the primary series of tests because they have a specific gravity of 1.1, can be ground and sorted into different sizes, and the particles have about the same angular shape as natural eolian material
sures on Mmrs (•1 to 10 mb) require substantially stronger winds to move particles than on Earth. The basic physics of wind-blown particles is pre-
(sand and loess). Because the shape of the ground walnut shells is comparable to natural aeolian material, the lift and drag coefficients should
sented in the classic work by Bagnold (1941), but
also be comparable.
curve of Greeley et al. these
parameters
into
(1974)
does not take
account.
the relative
importance
of individual
such as lift
coefficients and cohesion is poorly
known. Yet, an understanding is critical for extrapolations
parameters
of these parameters to Mars. Although
An important factor in particle
threshold is
the effect of cohesion from adsorbed moisture and from other surface forces. This effect increases
someexperiments have been conducted at 1 bar
with sinroller particles
which take these parameters into account (Iversen et al.., 1976), current extrapolations to Mars
mass ratio increases. To determine if cohesion from moisture is the primary cause for the "upturn" in the threshold curve for small particles
produce widely divergent results because of uncertainties in knowledge of forces on small particles at low pressure. Wind
Tunnel
Simulations
lished
at
NASA-Ames
Research
Center
in
which
threshold experiments can be conducted under conditions approximating those of Mars except for the differences in gravity and atmospheric composition. Experiments were run by placing a patch of grains in the tunnel test section and, under the desired atmospheric pressure (5.3 mb for the TABLE
MATERIALS MATERIAL
and to assess the effect at low pressure, several particle tests were conducted in which the samples were dried by heating and then tested in the tunnel.
Because of the uncertainties in particle movement on Mars, a low pressure wind tunnel was estab-
SPECIFIC
since the surface area-to-
The
test
bed
of
the
tunnel
was heated
Because not all parameters involved in martian eolian processes can be simulated in wind tunnel tests on Earth, it is necessary to use a combination of theory and wind tunnel results for extrapolation to Mars. Before the extrapolation is made, however, as much of the theory as possible should be tested. The expressions derived by 1
USED FOR THRESHOLD TESTS
GRAVITY
SIZE
RANGE, •4m
SHAPE
,
Walnut shell Calcium carbonate
1.1 2.7
Glass microspheres
2.7
Talc
3.0
to
the temperature that kept the sample dry but did not affect the boundary layer.
20-700 20-500 40-300 5-
20
angular subrhombohedra
1
spherical
angular
to platy
Greeley
et
al.:
Wind Friction
Speeds
419
Iversen et al., (1976) for extrapolation to Mars take into account interparticle forces, surface roughness, lift coefficients, and other parameters, but are based on wind tunnel tests performed
at
1 bar
and
contain
uncertainties
as
-
110
100
30(
to
25(
the behavior of particles under low pressure. These expressions were tested against the wind tunnel experiments performed at 5.3 mb (fig. 1) and found to be in good agreement, particularly in the critical particle size range of 30 to 200 •m. The cause of the divergence of the test results and the theory for large particles, however, is unresolved. Future experiments will focus on both larger and smaller particles than those used in these experiments.
7o
20(
•
2.5mb 60 "•
/• 15( 5o •
•:
50mb
•
10.0mb
4o :>•
1 o(
50
Results
and
Conclusions --
Figure 1 shows threshold results performed at low pressure, but with Earth air; figure 2 shows
threshold
wind
friction
velocities
o
to
Mars
based
on the
results
of
for
Mars:
over a flat
1.
Minimum threshold friction
layer)
velocity
conditions smooth this cor-
stream (above the boundary
of about 125 m/sec.
Corres-
ponding wind speeds at the height of the Viking meteorology instrument would be about half the free stream wind speed. These values could be substantially lower, however, depending on the nature of the surface. For example, if large non-erodible
present,
elements
such
then the same V,•
as
cobbles
I
III
o
0
1ooo
(Vgt) basedon atmospheric modelsby Pollack (1976); Case 1 is for winds blowing
cobbles
minimum V,t is about2.5 m/sec. Usingthe
responds to a free
I
smooth surface composed of erod-
ible grains, Case 2 is for a surface containing
For a
of 5 rob, the
expressions for martian atmospheric (Pollack et al., 1976a) for a flat, surface composed of loose particles,
I
(above the boundary layer) wind velocities
for
pressure
I
scales on right are equivalent free stream et al.
surface
I
Figure 3. Martian particle threshold curves as a function of particle size at three pressures. Scale on left is V,• in cm/sec, two
Iversen
speed.
I I • •1 1oo
et al. (1976) using the appropriate values for kinematic viscosity and density for the martian atmosphere. Several results are significant
nominal martian
I
PARTICLE DIAMETER,
four atmospheric pressures, demonstrating the dependence of threshold on pressure. Figure 3 is an extrapolation of the wind tunnel results
I
lO
10
are
could be generated
by windsone third to oneChalfthat given
and small
boulders.
above (fig. 3). These minimum free stream winds are significantly higher than typical speeds on Mars obtained from general circulation calculations; however, such winds are predicted to occur occasionally (Pollack et al., 1976b). 2. Small particle threshold. Sagan and Baghold (1975) recently extrapolated particle threshold velocities to Mars based on experiments of cohesion-free particle transport. They suggested that particle cohesion due to impact vitrification, vacuum sintering, and adsorbed thin films of water
might
particles
be absent
on Mars,
and that
small
(• 1 •m) might be more easily moved than
100 •m particles. Low pressure wind tunnel results performed with dried samples and in the ab-
7 mb
sence of impact
5.3
ing still show an "upturn" in the threshold curve for small particles (fig. 1). Thus, if grain
6.5
movement is observed on the martian surface,
vitrification
and vacuum sinter-
minimum V,t at 5 mbis 2.5 m/secand wind speeds near the s6rface on the order of 25 to 75 m/sec should be expected. These estimates are based on particle threshold taking place on a flat surface; local topography such as raised-rim
7.9
10.5
craters
could initiate
threshold
at
lower wind
speeds as a result of local vortiCies, cussed by Greeley et. al. (1974).
3. 2 10
I
I
I
I
I I I I I
I
I
•
• • • • •1
100
1000
PARTICLE DIAMETER,/•m
on
pressure.
velocities
The particle
size
most easily moved (at a•m minimum Mars appears to be about 160 (fig. V•f) oron nearly twice as large as the optimum particle
on Earth.
size
Whensaltation begins, both larger
and smaller particles
Figure 2. Threshold curves for particles obtained in the wind tunnel for four pressures, showing the dependence of threshold
Optimumpa.rticle size.
as dis-
will
be set into motion.
The values of threshold speeds presented here represent the first series of experiments performedin a large, low pressure, boundary
420
Greeley
et al.:
layer wind tunnel and should be regarded as preliminary. Further refinements of the threshold
curve
and larger
and
particles
Acknowledgement.
extension
to
both
smaller
are in progress.
This work was supported
Re ferences
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